The present invention relates to a welding technique and particularly to a method for regulating the parameters of thermal cycles in electroslag welding.
The present invention may be broadly utilized in welding thick-walled structures.
Its most favorable utilization is for making welded structures from low-alloy normalized and thermally strengthened steels intended for use at subzero temperatures.
At the present time, the electroslag welding technique is widely employed for making structures of large thickness.
Under the conditions of the electroslag process, liberation of heat takes place when an electric current passes through the slag bath. This heat is used for the melting of the welding electrode, edges of an article being welded and flux. This entails loss of a considerable amount of heat. The loss of heat results from heat dissipation in the mass of the article being welded, slide blocks, and heat radiation from the slag bath surface. A larger portion of the thermal balance of the electroslag process (50%) is caused by the heat absorbed by the mass of the article being welded. In this connection the electroslag process is accompanied by retarded heating and cooling of the metal of articles being welded as well as with its being heated above the full re-crystallization temperature A.sub.3 for prolonged periods of time. This promotes development of substantial structural heterogeneity of welded joints. An adverse change of mechanical properties is manifested in a decreased resistance to brittle failure, and for thermally strengthened steels, additionally in a loss of strength.
Due to above mentioned facts, the area of application of the electroslag process is limited in welding structures intended for use at above zero temperatures, or those subject to subsequent high-temperature thermal treatment-normalizing.
Normalizing is aimed at improving the structure and mechanical properties of welded joints.
Realization of thermal treatment of this kind requires application of special heating devices. For example furnaces of substantial dimensions, and is prolonging of the production process. This makes the production of welded structures more expensive.
In a number of cases, normalizing may cause intolerable deformations and this makes its realization impossible. Therefore a full number of welded structures have to be produced by using less productive welding methods, such as automatic multilayer submerged arc welding.
Electroslag welding of thermally strengthened steels is accompanied with a particularly unfavorable reaction to the thermal cycle of welding. Along with a reduced resistance to brittle failure, weakening of welded joints is observed. Subsequent normalizing of such joints is not possible, because heating above the tempering temperature of the base metal at thermal strengthening is not feasible. Therefore, up to the recent time electroslag welding was not used for producing structures from thermally strengthened steels.
The required properties of welded joints in thermally strengthened steels were obtained by way of reducing the heat input of arc welding methods. This made the welding process more complicated because of an increased number of welding seams.
By regulating (reducing) the heat input of automatic submerged arc welding it is possible to a certain extent to influence the most important parameters of the thermal cycle of welding, heating rate, duration of keeping steel at higher than preset temperatures, and cooling rate. Consequently introducing favorable changes in the properties of welded joints.
In electroslag welding it is practically impossible to regulate the thermal cycle parameters by altering the welding conditions.
Up to now a number of methods have been proposed for improving the electroslag welding process with the aim of reducing negative influences on the properties of welded joints.
There is a known method of electroslag welding in which the electroslag welding is followed by local or total normalizing of welded joints, performed for the purpose of making them uniformly strong. However, in a number of cases, especially in structures made from low-alloy thermally strengthened steels, realization of high-temperature thermal treatment is impossible.
Also known is an electroslag welding method for regulating the thermal cycle by way of accompanying cooling.
According to this method in the course of electroslag welding, a welded joint is cool by means of a special device consisting of a seam forming slide block whose bottom portion has several rows of holes for the delivery of cooling medium.
As a result, the regulation of thermal cycle parameters and consequently that of the welded joint properties is possible only during the stage of cooling, from the minimum temperature of 950.degree. C., after heated metal appears from under the slide blocks. Actually, only one parameter of the thermal cycle is being regulated that is a cooling rate. Therefore, this method and device have the following disadvantages: (1) it fails to regulate thermal cycle parameters at the heating stage (heating rate Wh and t'--the duration of keeping heated metal at a temperature higher than A.sub.3):(2) increased intensity of heat dissipation from the welded joint. A higher rate of cooling the joint is possible only after the heated metal appears from under the seam forming slide blocks. Parameter t"--the duration of time the heated metal is kept at a temperature higher than A.sub.3 also remains non-regulated during cooling. These disadvantages lead to a situation where the total duration (t=t'+t") of keeping the heated metal at a temperature higher than A.sub.3 is not changed substantially when applying the known welding method as compared with conventional electroslag process technique, because t' and t" remain actually unchanged.
This leads to an intensive growth of the austenite grain with a resultant reduction in resilience especially of the seam metal and that of the seam-adjacent section of the thermal influence zone.
The above mentioned device is also not able to regulate the welding thermal cycle parameters pertaining to the separate sections of the thermal influence zone and of differentiating the cooling rate into temperature intervals within the given sections of the thermal influence zone. This disadvantage is particularly detrimental for thermally strengthened steels.
There are other methods of electroslag welding, which along with further improvements for better quality of welded joints also provide for higher productivity of the welding process.
Worthy of noting among these improvements are the reduction of the welding gap, the additional introduction of powdery adding material into the slag bath, higher welding electrode feeding speed, the additional heating of the electrode from a self-contained electric current source, the regulation of heat distribution and temperature in the slag bath, and the application of 5 mm diameter welding wire in welding process.
The above improvements, although speeding up the welding rate 1.5-2 times promote proportional reduction in the amount of heat brought to the welded joint. However, the heat input at doubling the welding speed as compared with the conventional technique still remains at a high level. Due to this fact, application of the above improvements in the electroslag welding would not be able to change the thermal cycle parameters within the limits approximate to an optimum level to ensure structural and mechanical uniformity of welded joints.
The object of the present invention is to eliminate the above disadvantages.
Other objects of the invention are to improve the metal structure in the seam and in the thermal influence zone, and production of practically equal mechanical properties of the welded joint and base metal.
Another object of the invention is opening a way for the possible application of the electroslag process for welding thermally strengthened steels.
Among other objects of the present invention that should be noted is a possibility of doing away with subsequent normalizing of welded joints.
The present invention has as its aim a method for regulating the parameters of the thermal cycle in electroslag welding. This would ensure practically equal properties of metal on the seam and in the thermal influence zone of the welded joints and base metal.
This task is accomplished by a method for regulating the parameters of the thermal cycles in electroslag welding. In this invention, the entire area of sections of the thermally influenced zone are simultaneously. The area is limited by the isotherms of maximum heating temperatures within an interval from temperatures close to the melting temperature up to austenite transformation temperature A.sub.1. The cooling is effected so that the cooling rate increases in the direction moving away from the sections with maximum heating temperatures close to the melting temperature towards sections with maximum temperature A.sub.1.
The thus proposed method for regulating parameters of thermal cycles in electroslag welding provides:
desired thermal cycle parameters during heating and cooling, viz., cooling and heating rates and the time during which the metal is subjected to a temperature higher than the heating temperature A.sub.3 and the extent of time that the metal will remain at temperatures higher than heating temperature A.sub.3 ;
higher degree of structural and mechanical uniformity of metal in the seam and the thermal influence zone and base metal.
The given method makes it possible to do away with subsequent normalizing of welded joints made by way of the electroslag process.
A further possibility arises for replacing the automatic multilayer submerged arc welding with the elctroslag process featuring regulation of thermal cycles.
Reduction in the number of seams made, as compared with multilayer welding, improves technical and economic characteristics of the production of welded structures.
Introduction of the electroslag welding technique, with regulation of thermal cycles, into the production of gas and petrochemical equipment permits an increase of three to four times productivity of the welding process. There is also a reduction in the number of weld seam defects from 12 to 3%.
The portions of the heated zone which have an identical maximum heating temperature, should preferalby be cooled in the temperature range of liberation of structurally free ferrite at a rate calculated to suppress such liberation, and in the temperature range of minimal austenite stability at a rate calculated to permit the austenite to disintegrate into desired structural components.
This assists in reducing the content of low-strength ferrite phase in the welded joint structure and in increasing the welded joint strength. Disintegration of austenite in the given area contributes to obtaining the required mechanical properties of welded joints: resilience, strength characteristics, etc.
Various rates of cooling the sections of metal at a seam and a zone subject to the heating effect are obtained by subjecting them to the effects of mediums with different heat transfer (heat-exchange) coefficients or by altering the cooling medium flow rate.
This allows the predetermined relationship to exist between the absolute values of the cooling rates. This has the purpose of creating austenite disintegration products similar in composition at the cooling stage in accordance with the welding thermal cycle.
For the purpose of carrying the above described welding method into effect there is a special device. The device is comprised of a welding seam forming a slide block with passages for the delivery of a cooling medium. The slide block is connected with a drive for its progressive motion and carries jets that have passages for the delivery of a cooling medium. It also has spray nozzles for the delivery of a cooling medium onto the sections of the welding seam and a zone subject to the heating effect. According to the invention, the jets are secured on both side faces of the slide block, while the outlet nozzles of the jets are so constructed that the cooling medium flowing out from these nozzles would cool down the entire surface at the metal of seam and a zone subject to the heating effect. The surface is thus limited by isotherms of maximum heating temperatures within an interval from temperatures close to the melting temperature up to temperature A.sub.1.
This brings about the optimization of the most important parameters of the heating and cooling stages. These thermal cycles are in electroslag welding (heating and cooling rates, duration of keeping at a temperature higher than temperature A.sub.3 when heating and cooling) for the purpose of gaining the maximum structural and mechanical uniformity of welded joints with respect to the base metal in the course of welding.
It is necessary to provide the side and bottom faces of the slide block with bosses having passages permitting the flow of a cooling medium. The flow comes in from the slide block into the jets; intended for attaching the jets.
This opens the way for interconnecting the cooling medium delivery passages in the slide block and in the jets and in addition feeds the jets with cooling medium. For example water, after it has passed through the slide blocks, increases in temperature and this aids in the arrangement of the variable cooling rate in the section of a zone subject to the heating effect.